Heretofore, in the production of vehicles such as cars, etc., reinforcement has been applied to steel plates for use in the production of the body thereof, for example, outer plates having a relatively large surface area, a flat form, and a small thickness, such as a roof, a fender, a hood, a trunk, a quarter panel, and a door, to provide increased strength, since it is necessary for them to have appropriate stiffness against forces on the outside surfaces thereof. This reinforcement has usually been accomplished by bonding inner plates comprising metallic reinforcements to the steel plates by spot welding or by the use of adhesives. This method, however, has disadvantages, in that the weight of the inner plates is relatively large, increasing the total weight of the body and production costs, and additional complicated steps for applying the inner plates are necessary.
It is also known that polymeric materials, such as asphalt rubber, epoxy resins, acrylic resins, phenol resins, and unsaturated polyester resins, can be coated on the back of such plates in a fairly large thickness and over a fairly broad surface to achieve both the purposes of prevention by vibration and enhancement of the strength of the plates. This method offers advantages in that relatively large reinforcing effects can be obtained by selecting polymeric materials having high modulus of elasticity in tension and adhesive force, and coating such polymeric materials in an appropriately large thickness. Therefore, this method is advantageous over the method using metallic reinforcements with respect to the weight and ease of operation.
Almost all polymeric materials having high modulus of elasticity in tension and adhesive force used according to the above method are thermosetting resins which are generally subjected to three-dimensional cross-linking. The use of such thermosetting resins, however, gives rise to the crucial problem that the residual stress resulting from shrinkage of the thermosetting resin after the hardening thereof produces undesirable strain (and resulting depressions) in the outer plate, deteriorating the surface evenness of the body, and therefore significantly deteriorating the product value.
It has therefore been desired to develop a novel and useful method for the reinforcement of metal plates, which uses thermosetting resins, thus offering advantages over metallic reinforcements, and which also permits obtaining sufficiently great reinforcing effects while preventing the formation of strain (and resulting depressions) in the metal plate, resulting from the shrinkage of the thermosetting resin after the hardening thereof.